1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting and receiving packet data in a wireless communication system, and in particular, to an apparatus and method for transmitting and receiving packet data in a wireless communication system using multiple antennas.
2. Description of the Related Art
In general, a mobile communication system is the most typical wireless communication system. The mobile communication system has developed based on voice communication. However, with the increasing user demand for and the rapid progress of communication technology, the mobile communication system is now evolving into an advanced system capable of transmitting high-speed, high-quality multimedia data. The mobile communication system is roughly classified into a synchronous system and an asynchronous system. As for the asynchronous system, many researches and standardization works on High Speed Downlink Packet Access (HSDPA) are being conducted in 3rd Generation Partnership Project (3GPP). In addition, as for the synchronous system, many researches and standardization works on 1x Evolution-Data Voice (EV-DV) are being conducted in 3rd Generation Partnership Project 2 (3GPP2). The researches and standardization works are the representative attempts to find a solution for a high-speed (2 Mbps or higher), high-quality radio packet data transmission service in a 3rd generation (3G) mobile communication system.
A 4th generation (4G) mobile communication system aims at providing a higher-speed, higher-quality multimedia service compared with the multimedia service provided in the 3G mobile communication system. As a technique for providing a high-speed, high-quality data service in the wireless communication system, a multi-antenna technique is now under serious discussion. A description will now be made of the multi-antenna technique.
The multi-antenna technique used for a transmitter and a receiver of the wireless communication system adopts Multi-Input Multi-Output (MIMO) technology. It is known that the multi-antenna technique transmits data using multiple antennas, thereby increasing a data rate in linear proportion to the number of transmission/reception antennas without an additional increase in frequency bandwidth. The multi-antenna technique will be described in more detail herein below. A multi-antenna system has two or more transmission antennas and two or more reception antennas. The number of the reception antennas must be greater than or equal to the number of transmission data streams. In this system, a transmission side can transmit data through the antennas in parallel. Then a reception side can receive the transmitted data using a plurality of antennas. Therefore, the system can increase a data rate without an additional increase in bandwidth.
The multi-transmission/reception antenna technique applied to the system can be classified into a spatial diversity technique and a spatial domain multiplexing technique according to the number of data streams transmitted simultaneously.
In the spatial diversity technique, a transmitter transmits only one stream. The spatial diversity technique has been developed to suppress a deterioration of link performance due to the fading occurring in a mobile communication channel. Therefore, the spatial diversity technique, as it transmits only one stream, is appropriate for the service whose allowable delay time for transmission data is limited, such as voice call, video call, and broadcasting services. On the contrary, the spatial domain multiplexing technique transmits multiple streams. The spatial domain multiplexing technique has been developed to increase a data rate, guaranteeing constant link performance. Therefore, compared with the spatial diversity technique, the spatial domain multiplexing technique is suitable for a packet data service having a longer allowable delay time.
A multi-transmission/reception antenna system can be classified into an open loop system and a closed-loop system according to whether channel status information (CSI) is fed back from a receiver. The open loop multi-antenna system is used when a transmitter receives no CSI being fed back from the receiver, and the closed-loop multi-antenna system is used when the transmitter receives a CSI being fed back from the receiver.
The current 3G wireless packet data communication system such as HSDPA and 1xEV-DV, which are under standardization or commercialization, uses a link adaptation technique such as an Adaptive Modulation and Coding (AMC) technique and a scheduling resource management technique to improve transmission efficiency. The link adaptation technique receives partial channel status information being fed back from a receiver and applies an appropriate modulation and coding scheme (MCS) determined to be the most effective according to the received partial channel status information. Therefore, the multi-antenna technique classified as the closed-loop spatial domain multiplexing technique is most effective in the wireless packet data communication system.
However, the spatial domain multiplexing technique is not always applicable to the multi-antenna system. When multiple streams are transmitted over the channels having a very high spatial correlation or a very low signal-to-noise ratio (SNR), an error rate increases, disabling stable communication. Therefore, a transmitter requires an adaptive multi-antenna technique for selecting an effective technique based on a channel status report from a receiver.
It is known that in communication between one multi-antenna transmitter and one multi-antenna receiver, the data capacity serviceable through the spatial domain multiplexing technique increase in linear proportion to the number of transmission/reception antennas. However, the recent research has proven that when communication is performed between one transmitter and a plurality of receivers like in the forward link, the conventional spatial domain multiplexing technique cannot obtain the linear increase in the data capacity and a spatial domain multiple access technique can overcome the difficulty. A multiple access technique simultaneously transmits data to a plurality of receivers. The multiple access technique includes Time Domain Multiple Access (TDMA) for dividing time resources to secure channels for multiple users, Frequency Domain Multiple Access (FDMA) for dividing frequency resources, and Code Domain Multiple Access (CDMA) for dividing code resources. However, the spatial domain multiple access technique divides spatial resources, sharing the time, frequency, and code resources, thereby to secure multiple user channels.
However, the spatial domain multiple access technique is not always applicable to the multi-antenna system. Like the spatial domain multiplexing technique, the spatial domain multiple access technique is applicable or not applicable to the multi-antenna system depending on the channel status. For example, if a correlation between channels for two users is high, the two users cannot be spatially separated. In this case, the use of the spatial domain multiple access technique increases an error rate, making it impossible to perform stable communication. Therefore, in order to improve the data transmission capacity through the multi-antenna system, there is a need for a technique for adaptively selecting one of the spatial diversity technique, the spatial domain multiplexing technique, and the spatial domain multiple access technique according to channel status.
As described above, the multi-antenna technique is roughly classified into the spatial diversity technique and the spatial domain multiplexing technique. The spatial diversity technique includes a transmission diversity technique using multiple antennas for a transmitter, and a reception diversity technique using multiple antennas for a receiver. The reception diversity technique can increase an average of received SNRs and reduce dispersion of the received SNRs. Therefore, the reception diversity technique, when applied to packet data communication, contributes to an increase in the data rate. However, the transmission diversity technique, in which a transmitter does not take the CSI into account, uniformly distributes the total transmission power to the antennas, making it difficult to expect an improvement in the average of the received SNRs. The transmission diversity technique merely reduces the dispersion of the received SNRs. In packet data communication for which a scheduler is used, a transmitter selects a receiver with the highest transmission efficiency among several receivers every predetermined time and transmits data to the selected receiver. Therefore, for the same average of the received SNRs, a technique of increasing the dispersion can improve the data transmission efficiency. As a result, the transmission diversity technique in which the CSI is not taken into account is rather inferior to the single-antenna technique in terms of the data transmission efficiency. However, the spatial diversity technique in which the CSI is taken into account can improve the average of the received SNRs, contributing to an increase in the data transmission efficiency.
The typical transmission diversity technique in which the CSI is not taken into account is Space Time Coding (STC), and the transmission diversity technique in which the CSI is taken into account includes Selective Transmit Diversity (STD) and Transmit Adaptive Array (TxAA). In STD, a receiver informs a transmitter of an antenna having the best channel status among multiple transmission antennas and then the transmitter transmits signals through the antenna. In TxAA, a receiver transmits a complex response of an estimated channel to a transmitter and then the transmitter performs beamforming so that a base station maximizes an SNR. In the transmission diversity technique in which the CSI is taken into account, the CSI taken into account by the transmitter is information indicating an antenna having the best channel status, for STD, and a complex response of a channel, for TxAA.
Also, the spatial domain multiplexing technique is classified into a technique in which the CSI is not taken into account and another technique in which the CSI is taken into account. The CSI-independent technique cannot but transmit the same amount of data all over the transmission antennas. Therefore, a receiver requires a method for minimizing an error rate in this situation. On the contrary, the CSI-dependent technique can transmit the different amount of data through each transmission antenna. The CSI-dependent technique includes a Per Antenna Rate Control (PARC) technique. A PARC transmitter receives a CSI for each antenna, being fed back from a receiver. Based on the feedback CSI, the PARC transmitter selects an MCS for transmission of a greater amount of data for an antenna having the better channel status, and selects an MCS for transmission of a less amount of data for an antenna having the poorer channel status. The PARC transmitter will now be described in more detail.
FIG. 1 is a block diagram illustrating an exemplary internal structure of a PARC transmitter. With reference to FIG. 1, a detailed description will now be made of an internal structure and operation of a PARC transmitter.
Although two transmission antennas are provided in FIG. 1 by way of example, the number of transmission antennas is extensible. However, a receiver should satisfy the prerequisite that the number of reception antennas should not be less than the number of the transmission antennas in order to distinguish different streams transmitted from different transmission antennas. A description of a method for measuring, by a receiver, channel quality information (CQI) for each transmission antenna and receiving the measured CQI will not be given with reference to FIG. 1. In other words, it will be assumed that the transmitter of FIG. 1 receives a CQI for each antenna from the receiver.
As illustrated in FIG. 1, a CQI for each antenna is input to a feedback signal receiver 101. The feedback signal receiver 101 delivers the CQI for each antenna to a demultiplexer (DEMUX) 102 and an AMC block 103. The demultiplexer 102 receives a user data stream to be transmitted, and demultiplexes the transmission user data stream into two substreams according to the information received from the feedback signal receiver 101 such that the substreams are transmitted through the corresponding antennas. The demultiplexing process performs demultiplexing such that the greater amount of information is provided to an antenna having the better channel status. Herein, a stream output from the demultiplexer 102 will be referred to as a substream. The substreams separated for the individual antennas are input to independent AMC blocks 103 and 104, respectively. The AMC blocks 103 and 104 perform modulation and coding appropriate for the channel status of the corresponding transmission antennas according to the information received from the feedback signal receiver 101. The substreams, after being subject to the modulation and coding process, are transmitted to the receiver through the transmission antennas 105 and 106.
Theoretically, to maximize the data transmission efficiency, the spatial domain multiplexing technique adapts a method for performing singular value decomposition (SVD), with the channel status known to both the transmitter and the receiver, to form multiple interference-free subchannels from a multi-antenna channel, and selecting appropriate modulation scheme, coding scheme, and allocated power for each subchannel. This method is called “SVD MIMO.” However, in the actual mobile communication environment, because the transmitter cannot accurately estimate transmission channels, there is a limitation in realizing SVD MIMO. Therefore, a Per Stream Rate Control (PSRC) technique has been proposed as a technique that restrictively uses SVD MIMO.
In the PSRC technique for restrictively implementing SVD MIMO, a receiver estimates a multi-antenna channel and determines a preprocessed matrix to be used in a transmitter according to the estimation result. If there is no limitation in the amount of feedback information and there is no error occurring in the feedback process, the preprocessed matrix will be a unitary matrix obtained by performing SVD on a multi-antenna channel expressed in the form of a matrix. In reality, however, this cannot be realized, because the feedback information is quantized information. Therefore, the PSRC technique uses a method in which a receiver selects the most preferred preprocessed matrix among a predetermined number of candidate preprocessed matrixes, and informs a transmitter of the selected preprocessed matrix.
In this method, the transmitter multiplies a transmission signal by the selected preprocessed matrix and transmits the resultant data, and multiple beams are formed in this process. Therefore, in the PSRC system, a receiver informs a transmitter of a multi-beam forming method appropriate for its channel so that the transmitter forms multiple beams appropriate for the corresponding user. Based on the information, the transmitter allocates data streams to the formed beams on one-to-one basis to simultaneously transmit a plurality of data streams, realizing the spatial domain multiplexing technique. Because the transmitter allocates data streams to the formed beams before transmission, the receiver additionally transmits channel status information of each beam to the transmitter so that the transmitter can adaptively manage the amount of information for the data transmitted through each beam. That is, the transmitter selects an MCS such that the greater amount of information can be transmitted through a beam having the better channel status, and selects an MCS such that the less amount of information can be transmitted through a beam having the poorer channel status. A description will now be made of the PSRC transmitter.
FIG. 2 is a block diagram illustrating an exemplary internal structure of a PSRC transmitter. With reference to FIG. 2, a detailed description will now be made of an internal structure and operation of a PSRC transmitter.
Although two transmission antennas are provided in FIG. 2 by way of example, the number of transmission antennas is extensible. However, a receiver should satisfy the prerequisite that the number of reception antennas should not be less than the number of the transmission antennas in order to distinguish different streams transmitted from different transmission antennas. A description of a method for measuring, by a receiver, a CQI for each transmission antenna and receiving the measured CQI will not be given with reference to FIG. 2. In other words, it will be assumed that the transmitter of FIG. 2 receives a CQI for each antenna from the receiver.
A feedback signal receiver 201 of the transmitter receives a beamforming weight and CQI information for each beam, and provides them to a demultiplexer 202, AMC blocks 203 and 204, and beamformers 205 and 206. The demultiplexer 202 receives a user data stream to be transmitted, and demultiplexes the user data stream into as many substreams as the number of antennas such that the substreams are transmitted through the beams. The beam demultiplexing in the demultiplexing process is performed such that the greater amount of information is provided to an antenna having the better channel status. The separated substreams are input to the independent AMC blocks 203 and 204, respectively. Each of the AMC blocks 203 and 204 receives AMC information for a corresponding beam, provided from the feedback signal receiver 201, and modulates and codes the corresponding substream according thereto.
The substreams, after being subject to the modulation and coding process, are transmitted to the beam formers 205 and 206, respectively, and the beam formers 205 and 206 form beams using the weight information provided from the feedback signal receiver 201. The signals output from the beam formers 205 and 206 are input to adders 207 and 208, added therein to the input signals, and then transmitted to a receiver through transmission antennas 209 and 210.
To determine a beamforming method, each receiver feeds back a beamforming weight to the transmitter. An expression method of the feedback beamforming weight follows that of TxAA. Assuming that a transmission antenna #1 is a reference antenna, a TxAA receiver feeds back a ratio of a channel status α1 for the reference antenna to a channel status α2 for a transmission antenna #2, to the transmitter. That is, the receiver feeds back α2/α1. However, because the amount of feedback information should be limited, a value of α2/α1 is subject to quantization. In TxAA mode #1, a phase value of α2/α1 is quantized with 2 bit, and in TxAA mode #2, a phase value of α2/α1 is quantized with 3 bits and a magnitude value thereof is quantized with 1 bit. A TxAA transmitter forms one beam based on the feedback information and transmits a data stream using the beam.
The PSRC transmitter creates two beams by forming one beam based on the feedback information and additionally forming another beam being orthogonal thereto, and transmits demultiplexed separate substreams with the two beams. Because the PSRC system forms beams based on the quantized feedback information in this manner, it cannot generate interference-free subchannels like SVD MIMO. In addition, because each user requires a beamforming method appropriate for its own channel status, there is a difficulty in extending the PSRC technique to the spatial domain multiple access technique.
The CSI-independent spatial diversity technique like STC contributes to a reduction in dispersion of received SNRs. A technique for reducing the dispersion of received SNRs is effective for the service that restrictively depends on the CSI and requires real-time transmission, such as voice call, video call, and broadcasting services. However, because packet data communication allows a time delay, it uses a scheduling technique and selects an AMC method based on the CSI. Therefore, the technique for reducing the dispersion of received SNRs is not appropriate for wireless packet data communication. For the same reason, the CSI-independent spatial domain multiplexing technique is also not appropriate for the wireless packet data communication.
The CSI-dependent multi-antenna technique has been developed to individually realize the spatial diversity technique or the spatial domain multiplexing technique. Therefore, in order to effectively manage the multi-antenna technique, the transmitter should be implemented such that it selects an appropriate one of the separate CSI-dependent techniques. In this case, the transmitter must transmit, to the receiver, additional information indicating a type of the multi-antenna technique in use, in the process of transmitting data. The transmitter consumes a part of available resource in the process of transmitting the additional information, decreasing transmission efficiency. In addition, the multi-antenna technique does not provide a method implemented for adaptively selecting one of the spatial diversity technique, the spatial domain multiplexing technique and the spatial domain multiple access technique according to the channel status. Accordingly, there is a demand for such a method.